A Synthetic Aperture Radar (SAR) takes advantage of digital signal processing to synthesize high-quality radar images from a plurality of lower quality images. Outputs of one moving antenna at different locations and times, or a plurality of spatially separated antennas, are synthesized together to achieve higher resolution images than would be possible from a single radar with the same aperture size.
Two types of SARs include a radar array having a plurality of antennas that are stationed at different locations on the ground and a moving radar carried by an aircraft or spacecraft. The preferred embodiment as described herein uses the latter. The terminology of "Synthetic Aperture Radar" or "SAR" will be used herein to describe a moving radar system on an aircraft or spacecraft.
In topographic mapping using a SAR system, the aircraft or spacecraft flies over a targeted region along a flight track to collect data. The single antenna serves as both the signal transmitter and the signal receiver. After the transmitted electromagnetic pulses hit the targeted area, scattered pulsed waves are generated in many directions. The antenna receives a portion of the backscatted energy from the pulses imaging the targeted area. The slant range position of a particular target in the area to be mapped can be determined by measuring the time delay between transmitting and receiving of a pulse. A SAR processor processes the received signals by digital or optical signal processing techniques. The SAR system can produce a high-resolution two-dimensional map of a targeted area based on the amplitude and phase information of the received "backscattered pulses". Such SAR systems are well known to the art. One well-recognized limitation thereof is the lack of information about the target height or elevation.
An interferometric SAR is a particular kind of SAR system having two spatially displaced antennas installed on an aircraft or spacecraft. One significant improvement of an interferometric SAR system over the single-antenna SAR system is its capability of obtaining the information on a third dimensional parameter of a target to thereby produce high-resolution 3D topographic images. Similar to a single-antenna SAR system, the aircraft or spacecraft in an interferometric SAR system flies along a flight track over a targeted area for mapping. Either one or both of the antennas can serve as transmitters to send coherent electromagnetic pulses to an targeted area. A portion of the backscattered pulses from the target area is received by both antennas.
The location of the aircraft or spacecraft is known from independent sources such as global positioning system (GPS), digital avionics data system (DAD) and/or inertial navigation units (INU). Thus two-dimensional images of the targeted area can be obtained by digital signal processing techniques based on the amplitude and phase information of the received backscattered pulses from each antenna. The two images thereof are nearly identical due to the relatively small spatial displacement of the two antennas with respect to the slant range from the targeted area to the aircraft or spacecraft.
Interferometric SAR technology takes advantage of the coherent interference of the two 2D images from the two antennas. In particular, SAR interferometry employs the phase difference resulting from the relatively small difference in the slant range from a point in the targeted area to the two antennas to extract data for a third dimensional parameter. That third parameter is preferably the elevation or the height variation of the targeted area. This elevation information allows a three-dimensional topographic map to be obtained.
In general, radar techniques for precision topographic maps have many advantages over traditional stereo photography, including all weather, day and night capability, potentially fast automated processing, and the potential to provide absolute location without using known ground reference points. Traditional radargrammetry is capable of generating three dimensional images by using two spatially separated antennas for detection and by extracting elevation information directly from the difference in the slant range of a targeted point to two antennas. The separation of the two antennas is known as the "baseline". A large baseline is required in order to achieve good image resolution in traditional radargrammetry, which increases the complexity in coregistration of the two radar images.
Interferometric SAR technology significantly improves the three dimensional mapping capability of the traditional radargrammetry by using the phase difference instead of slant range difference of two radar images. As known in the art, such a phase signal is sensitive to range changes of fractions of a wavelength, which is in a range of 10.sup.-2 m for a radio wave of several GHz. This property of phase enables SAR interferometry to achieve better accuracy than traditional radargrammetry in additional to other technical advantages. As a powerful tool for producing topographic maps, airborne interferometric topographic SAR is able to provide high resolution 3-dimensional maps with horizontal and vertical accuracy at the order of a meter.
There are several critical technical issues involved in interferometric SAR including:(1) accurate radar platform positions and interferometric baseline estimates; (2) Motion compensation in signal processing; (3) Preservation of relative signal phases in the processor; (4) An algorithm to determine the absolute phase; (5) A 3-dimensional location algorithm.
SAR interferometry uses the phase information, and hence requires several interferometric SAR techniques to work cooperatively. This phase processing is a rather complex process due to a number of facts. The irregular topography of the unknown targeted area can adversely affect the determination of the corresponding unambiguous interferometric phase in a consistent manner. More over, the actual flight track of the aircraft changes constantly and randomly due to various reasons. Therefore, the range will change accordingly which causes systematic phase variations that must be corrected appropriately. The detected signals are inherently noisy due to factors such as electromagnetic interference, receiver electronic noise, multi-pass reflection noise from the aircraft, and topographic profile of the targeted area. For these and other reasons, the techniques for phase determination in interferometric SAR must be sophisticated and sensitive as they are crucial to the fidelity and accuracy in topographic mapping.
In recognition of these problems and drawbacks, the present invention describes an integrated processing system for SAR interferometry. The present invention implements correction of radar platform motion deviations, interferogram coregistration of images from two antennas, and a spectral shifting for optimal interferogram correlation. These and other processes work together to achieve highly accurate and automated 3D topographic mapping.
One foundation for the present invention is an unique coordinate system, the SCH-system, upon which the radar platform motion compensation, many processing operations, and result reporting are based. The SCH-system is optimized for a particular point on the flight track of the radar platform and uses a radius of the curvature of the earth thereof that is a function of both that point and the heading of the flight track. The advantages of this SCH-system will become apparent in detailed descriptions hereinafter.
As stated above, phase determination is a key element in an interferometric SAR processing system. The present invention implements sophisticated processes to ensure the effectiveness, efficiency, accuracy, and robustness in determining the relative phase values in all image segments as well as the absolute phase of a particular image segment for the absolute phase values of the entire image. One unique feature of the algorithms for absolute phase determination is that the present invention can operate without known ground reference points to determine the absolute phase.
High-resolution three dimensional topographic mapping provides essential information for positioning. Such positioning has myriad uses including navigation, geological exploration, seismic studies and other fields. The ability to acquire this information may very well lead to new fields of art which are not yet even conceived!
The advantages, sophistication, and significance of the present invention will be more apparent in the light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.